In the current UBB implimention Active topics uses the search system; the search system has a variable set in the CP that allows only so many searches for a set period (as searches can be intensive on the server).
The user needs to wait for that timeout to run out, or you need to increase the timeframe:
CP -> Master Settings -> Feature Settings -> Search Tab -> Minimum Time Between Searches
In a later version Rick has said that he's going to rework the system to no longer use the search scripts.
